In the manufacture of devices using optical fibers, it is commonly required to heat the fibers in a controlled manner so that they may be spliced, coupled, shaped, annealed, tapered, diffused, expanded, flame polished, cleaned, or stripped of coatings. An electrical discharge is commonly employed for this purpose. This electrical discharge is known in the industry as an “arc.” However, according to some sources, a discharge of this current level is not a true arc, but a glow discharge that generates a hot plasma.
The arc is normally formed between the sharply pointed tips of a pair of electrodes, typically made of tungsten and spaced 1 mm to 10 mm apart. Larger electrode spacing is required for splicing multiple fibers at once (fiber ribbons), and for larger diameter fibers. The optical design of some splicers may also require the electrode spacing “gap” to be larger in order to prevent the electrodes from physically occluding the optical path.
The voltage applied to the electrodes may be DC (typically in conjunction with smaller electrode spacing) or AC (which allows a larger spacing between the electrode tips—up to 10 mm or more). The voltage required to initiate the discharge is determined by Paschen's Law, which relates the breakdown voltage of a gap between electrodes to a (complex and non-linear) function of the gas present in the gap (typically ordinary air), pressure, humidity, electrode shape, electrode material, and gap distance. Many of the parameters required to apply Paschen's Law to this system are not known, so little quantitative theoretical analysis of splicer arcs has been done. Typically, the initiating voltage is determined experimentally to be in the range of 5 kV to 30 kV.
FIG. 1 shows a schematic representation of a typical prior art fiber processing device using an electrical discharge as a heat source, known as a “fusion splicer”. This device has as its primary purpose the splicing of two fiber ends together, but may also be used for other operations, such as tapering. The two fiber ends are held by fixtures (3,4) which can be positioned in at least two axes each. An Arc Discharging Unit (5) provides controlled high voltage to two pointed tungsten alloy electrodes (1,2). A programmable control unit (6) positions the fibers and controls the operation of the Arc Discharging Unit. Typically, these mechanisms are used in conjunction with one or more cameras and associated optics (not shown) to locate the fibers for positioning and to analyze the resulting splice quality.
Once the arc has been initiated, sustained ionization of the plasma in the discharge requires a lower voltage than initially applied. The impedance (ratio of applied voltage to current) of the plasma as a circuit element is difficult to predict. Splicer arcs are even suspected to exhibit negative incremental impedance at some frequencies and current levels. These characteristics make “constant voltage” operation of a splicer arc very difficult to achieve. Therefore, most such systems are controlled to provide a constant average current. This correlates in a reasonably predictable way with the observed power delivered to the discharge and the resulting temperature of the fibers.
However, the accuracy, precision, and repeatability of such control methods is subject to many uncontrolled factors. Air pressure, humidity, air temperature, electrode spacing, electrode cleanliness, and electrode geometry produce unacceptably large changes in the temperature reached at the working surface of the fibers. The electrodes oxidize away during use, which expands the gap between the electrodes, blunts their points, and contaminates their emitting surfaces.
As a result, various procedures have been developed to renormalize the relationship between the setpoint arc current and the resulting fiber temperature. These procedures normally consist of an “arc check” wherein the arc discharge is operated at various power levels, and the resulting distortion or incandescence of the fibers is observed by a camera to provide information used to recalibrate the system for atmospheric and electrode conditions. These procedures are unsatisfactory in many respects, as they consume time, electrode life, and optical fiber, while providing only a temporary and partial solution to the problem of changes in the fiber temperature.